Well first off - welcome!
Second, the best way to learn how to mod is to download a similar mod and see how it's made. I'd recommend taking a look at this one: http://www.gta4-mods.com/mobile/soli...ellphone-f5052
. You'll see that the author has included wdr and wtd files. wdr is for models, so those two are the actual 3D model of the phone. wtd files are for textures, so that's what gives it the gold look. Check out the ReadMe to see what archives these wdr and wtd files are in. You can open them up with SparkIV and extract PNG files, which you can then edit in Photoshop.
And of course you can do the same thing for skins - download any skin off the site and check out which files it replaces.